Environmental control system including ozone-destroying catalytic converter having anodized and washcoat layers

ABSTRACT

An aircraft environmental control system includes a catalytic converter having ozone-destroying capability. A surface of the catalytic converter is anodized to form an anodized layer, and the metal oxide layer is washcoated to form a washcoat layer. An ozone destroying catalyst is impregnated in the anodized and washcoat layers. The catalyst may include one or more metals. For example, a bimetallic catalyst may include a precious metal and a transition metal.

This application claims the benefit of provisional application No.60/101,140 filed on Sep. 18, 1998.

BACKGROUND OF THE INVENTION

The present invention relates to environmental control systems. Morespecifically, the present invention relates to an environmental controlsystem including an ozone-destroying catalytic converter.

A commercial aircraft usually includes an environmental control systemfor providing a stream of cooled, conditioned air to an aircraft cabin.A typical environmental control system receives compressed air such asbleed air from a compressor stage of an aircraft gas turbine engine,expands the compressed air in a cooling turbine and removes moisturefrom the compressed air via a water extractor.

Toxic ozone in the compressed air becomes an issue when an aircraft iscruising at altitudes that exceed 20,000 feet. To reduce the ozone to alevel within satisfactory limits, the environmental system is providedwith an ozone-destroying catalytic converter.

There are a number of desirable characteristics for an ozone-destroyingcatalytic converter of an aircraft. These characteristics include a)high efficiency of ozone conversion at bleed air operating temperature;b) good poison resistance from humidity, sulfur compounds, oil, dust,and the like, which may be present in the compressed air (for long lifeand minimum system overhaul and maintenance costs); c) light weight tominimize system parasitic load; d) high structural integrity of catalystsupport under extreme heat and/or vibration shock, which may ariseduring normal flight conditions (also for long life and minimum systemoverhaul and maintenance costs); and e) high mass transport efficiencywith low pressure drop.

An ozone-destroying catalytic converter with a metal core may bewashcoated with a slurry of a water-based silica sol and a refractorymetal to form an undercoat layer followed by an overcoat layer ofalumina oxide. Both layers may then be catalyzed directly by dipping thewashcoated core in a catalyst solution having strong acidity. However,the strongly acidity can cause corrosion of the metal core, especiallyif the core is made of aluminum.

The overcoat layer may be pre-catalyzed and then washcoated onto thecore. Using a pre-catalyzed layer can prevent corrosion during thecatalyzing process.

Applying the pre-catalyzed overcoat layer can be problematic. Forexample, it is difficult to control the uniformity of washcoat layerthickness. Unevenness of the layer thickness can cause a pressure dropacross the catalytic converter.

Another problem with the pre-catalyzed overcoat layer is poor catalystutilization efficiency. Washcoating the pre-catalyzed metal oxide canrender certain fractions of the catalytic site inaccessible due to theshielding of the binder material. Furthermore, the surface area providedby the undercoat is not utilized to extend the catalyst lifetime. Sincepoisons in the compressed air can reduce the efficiency of conversion,lifetime and efficiency of the catalytic converter is further reducedbecause of the poor catalyst utilization efficiency.

Another potential problem with water-based washcoat layers is itslimited mechanical durability. A catalytic converter for a commercialaircraft is subjected to high temperatures and large temperature swings(e.g., between 150° F. and 500° F.) during normal flight operation. Thecatalytic converter is also subjected to high vibrations during normalflight operation. These harsh conditions can cause the washcoat layer toflake off. Consequently, operating life of the catalytic converter isreduced.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, an ozone-destroyingcatalytic converter comprises a core; an anodized surface layer formedfrom a portion of the core; a washcoat layer on the anodized layer; andan ozone-destroying catalyst impregnated in the washcoat layer. Thecombination of the anodized and washcoat layers offers many advantages.The anodized layer provides a support for the catalyst and a corrosionbarrier that prevents a catalyzing reagent from attacking the coreduring catalyst impregnation. Therefore, the catalyst can be impregnatedafter formation of the washcoat layer to provide maximum catalystutilization and lifetime. The anodized layer significantly improves thebinding strength between the core and the washcoat layer, which allowsthe washcoat layer to withstand high temperatures, large temperatureswings and high vibrations such as those occurring during normalaircraft flight conditions. The anodized layer also provides additionalsurface area and, therefore, increases the efficiency of ozoneconversion and mass transport.

According to another aspect of the present invention, the washcoat layermay be formed by creating a slurry including a refractory metal oxideand an organosiloxane resin in monomeric or polymeric form. Therefractory metal oxide may be partially hydrated. The core is dipped inthe slurry and the resulting washcoat is dried. Such a slurry driesfaster than slurries that include water-based binders. The faster dryingallows the washcoat layer to be applied more uniformly than a washcoatlayer formed from a slurry that includes a water-based binder. Thus,thickness and roughness of the surface can be controlled better.

The dried washcoat is then cured and calcined. If the washcoat layer isapplied to an anodized layer of the core, cross-linking of the chemicalbonds between metal oxide particles, anodized surface and organosiloxaneresin occurs during the curing and calcination. This cross-linkingresults in a washcoat layer having significant mechanical and thermalstrength. Consequently, the washcoat layer is free from flaking.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an environmental control system includingan ozone-destroying catalytic converter according to the presentinvention;

FIG. 2 is an illustration of a cross-section of a surface of a core ofthe ozone-destroying catalytic converter, an anodized surface layerformed from the core, a washcoat layer on the anodized layer, and anozone-destroying catalyst impregnated in the anodized and washcoatlayers;

FIG. 3 is a perspective view of a portion of a plate-fin element for anozone-destroying catalytic converter; and

FIG. 4 is an illustration of a method of preparing a core of thecatalytic converter.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made to FIG. 1, which shows an aircraft environmentalcontrol system (“ECS”) 10 for an aircraft. The ECS 10 receivescompressed air such as bleed air from a compressor section of theaircraft's main engine. The ECS 10 includes at least one air-to-air heatexchanger (“HX”) 12 for cooling the compressed air to near-ambienttemperature; and an air conditioning system (“ACS”) 14 for conditioningthe air that was cooled in the air-to-air heat exchanger 12. The airconditioning system 14 may include an air cycle machine and a waterextractor for expanding the air and reducing moisture in the air. TheECS 10 supplies cooled, conditioned air to a cabin 16 or othercompartment of the aircraft.

The ECS 10 also includes a catalytic converter 18, which may be locatedin the belly of the aircraft, between the source of the compressed airand the air-to-air heat exchanger 12. The catalytic converter 18 ismounted inside a shell 20 through which the compressed air flows. Thecompressed air passes through the catalytic converter 18, which destroysozone in the compressed air.

A precooler, not shown, may be located upstream the catalytic converter18. The pre-cooler lowers the temperature of the compressed air prior toozone destruction.

FIG. 2 illustrates a cross-section of a surface of a portion of thecatalytic converter 18. The catalytic converter includes a core 22, ananodized surface layer 24 formed from a portion of the core 22, awashcoat layer 26 formed on the anodized layer 24, and anozone-destroying catalyst 28 impregnated in both the washcoat layer 26and the anodized layer 24 (the layers 24 and 26 and the catalyst 28 arenot shown to scale).

The anodized layer 24 has a thickness between about five to ten microns.The anodized layer 24 is dense at the interface with the core 22.However, the anodized layer 24 has a rough surface at the interface withthe washcoat layer 26. For a core 22 including aluminum fins about 7mils thick, the anodized layer 24 may have a preferred surface arearanging from 5 to 15 m²/gram, including the weight of the fins. Sincethe anodized layer 24 is integral with the core 22, it is difficult toaccurately determine the surface area of the anodized layer 24 excludingthe core 22.

The washcoat layer 26 is highly porous, having a preferred pore volumeranging from 0.2 to 0.9 cm³/gram, and an average pore size ranging from3 to 25 nm. The washcoat layer 26 has a large surface area. For analuminum core 22 having 7 mil thick fins that are anodized andwashcoated, the washcoat layer 26 has a preferred surface area rangingfrom 50 to 70 m²/gram including the weight of the fins, and a surfacearea ranging from 200 to 350 m²/gram excluding the weight of the fins.The washcoat layer has adjustable thickness between about five to sixtymicrons.

The catalyst 28 is impregnated into both the washcoat layer 26 and theanodized layer 24. Concentration of the catalyst 28 in the washcoatlayer 26 may be higher than concentration of the catalyst 28 in theanodized layer 24. For example, the catalyst concentration in thewashcoat layer 26 may be two to four times the catalyst concentration inthe anodized layer 24.

The catalyst 28 may be monometallic, or it may be bimetallic in the formof oxide with different valence states or in zero valance metallicstate. For a monometallic catalyst, a precious metal such as palladium(Pd) may be used. For a bimetallic catalyst, one of the metals may be aprecious metal such as palladium and the other one of the metals may bea transition metal such as nickel (Ni). In place of Pd, a metal from theprecious metal group including platinum (Pt), rhodium (Rh), gold (Au),iridium (Ir) or silver (Ag) may be chosen. Likewise, in place of Ni, ametal from the transition metal group including manganese (Mn), cobalt(Co), iron (Fe), or copper (Cu) may be used.

The core 22 provides a support for the layers 24 and 26. When a streamof air containing ozone is directed across the catalytic converter 18,the air interacts with the catalyst 28, resulting in the catalyticdecomposition of a majority of the ozone through the reaction,2O₃→3O_(2.) An air stream, filtered of ozone, flows past the converter18 to the heat exchanger 12.

The anodized layer 24 provides several functions. It provides a supportfor the catalyst. The anodized layer provides additional surface areafor better distribution of the catalyst, which improves overall ozonedestruction activity.

The anodized layer 24 also provides a corrosion barrier, which preventsa catalyzing reagent from attacking the core 22 and causing the metalcore to corrode during catalyst impregnation. During fabrication of thecatalytic converter 18, the anodized layer 24 allows the washcoat layer26 to be fully formed on the anodized layer 24 and then the catalyst 28to be impregnated, for example, by dipping the washcoated core 22 into abath containing the catalyzing reagent.

The anodized layer 24 is formed through electrochemical transformationof the surface of the core 22. Because the anodized layer 24 is anintegral part of the core 22, the anodized layer 24 significantlyimproves the binding strength between the core 22 and the washcoat layer26. The binding strength is further enhanced through chemicalcross-linking between the metal oxide of the anodized surface and aresin (i.e., an organosiloxane resin) and metal oxide in the washcoatduring washcoat formation. Therefore, the washcoat layer 26 has strongadhesion to the anodized layer 24 and is semi-flexible when the core 22deforms. As a result, the anodized layer 24 lessens the likelihood thatthe washcoat layer 26 will flake off when the catalytic converter 18 issubjected to high temperatures, large temperature swings and highvibrations during normal flight conditions.

The washcoat layer 26 also provides a high surface area support for thecatalyst. Additionally, the washcoat layer 26 can be applied to avariety of materials. Therefore, the washcoat layer 26 allows the core22 to be made of a lightweight metal such as aluminum or aluminum alloy.

In the alternative, the core may be formed of titanium, stainless steel,inconel, nickel alloy, cordierite, silicon nitride, alpha aluminumoxide, or other ceramic or composite materials. If the core 22 is formedof titanium or stainless steel, a thin oxide layer should be formed onthe core surface prior to washcoating.

The slurry for the washcoat layer 26, which includes a refractory metaloxide and an organosiloxane resin, can be applied more uniformly than aconventional washcoat layer including a water-based binder. Applying thewashcoat layer 26 uniformly reduces local buildups of the washcoat layer26 on air flow paths within the catalytic converter 18. The localbuildups can cause pressure drops within the catalytic converter 18 andredistribution of catalyst.

Because the washcoat layer 26 reduces the pressure drop inside thecatalytic converter 18, the catalytic converter 18 can be designed tohave a lower pressure drop than a conventional converter of similar sizeand configuration. Alternatively, the catalytic converter 18 may bedesigned to be longer yet provide the same pressure drop as aconventional converter. For example, the length of a core 22 having astraight channel air flow path can be increased. Increasing the air flowpath increases the exposure of the air to the catalyst and henceincreases the amount of ozone that is destroyed. Consequently, thelength of the catalytic converter 18 having a straight air flow path maybe longer than a conventional converter having a straight air flow pathand the same pressure drop.

Alternatively, a uniform washcoat layer 26 may allow a design of acatalytic converter 18 with a core 22 having a tortuous air flow path.The tortuous path increases turbulence of the compressed air, whichincreases mass transfer between the compressed air and the catalyst 28and hence increases ozone conversion efficiency. Consequently, thelength of the catalytic converter 18 having the tortuous air flow pathmay be shorter than a conventional converter having a straight air flowpath and the same or lower pressure drop.

Additionally, the better control of the coating uniformity and highermass transfer property allows the catalytic converter 18 to have a widerdiameter and a shorter length. Thus, the catalytic converter 18 may havethe shape of a short disk as opposed to a conventional elongated tube.The shorter, wider construction imposes a minimum pressure drop to theair flow into the ECS 10 when the air flow rate is high and increasesmass transfer rates that are beneficial for ozone destruction.

FIG. 3 shows a portion of an ozone-destroying catalytic converter 18that provides a tortuous air flow path. The catalytic converter 18includes a monolithic core 22 having a plurality of plate-fin elements23. The plate-fin elements 23 define a multitude of smalldirection-changing tortuous flow paths. The plate-fin elements 23 may bearranged in a tightly packed cylindrical configuration, as a pluralityof generally concentrically disposed rings. The plate-fin elements 23may be arranged in an axial succession of adjacent rows, with acorrugated configuration of generally rectangular profile. The outersurface of the plate-fin elements 23 carries the catalyst-impregnatedwashcoat and anodized layers 24 and 26.

Such a catalytic converter 18 may be constructed as follows. Offset finsare stamped into a plurality of aluminum sheets. A first sheet is rolledaround a tube to form an inner layer of the core. Additional layers areadded by rolling additional sheets, layer-by-layer, around the core 22.After the last layer has been added to the core, an uncorrugatedaluminum cover is wrapped around the outer layer. Then the core 22 isanodized, washcoated, impregnated with the catalyst and placed in theshell 20. The inner tube is blocked off to prevent the flow-through ofcompressed air. Thus, compressed air will flow longitudinally throughthe catalytic converter 18, between the inner tube and the outer coverand through the tortuous path created by the offset fins 23.

FIG. 4 shows a method of forming the catalyst-impregnated anodized andwashcoat layers 24 and 26 for the catalytic converter 18. First, thecore 22 is electro-anodized in an oxalic acid solution to form theanodized layer 24, as indicated at step 100. In place of an oxalic acidsolution, acids such as sulfuric acid, phosphoric acid, chromic acid ormalonic acid may be used. Because the anodized layer 24 is formed byelectro-oxidation of the metal of the core 22, the anodized layer 24 isintrinsically bound with the core 22. Because the anodized layer 24 isin the form of a metal oxide, it also provides additional surface areafor distributing the catalyst metals. Because the anodized layer 24 hasa different morphology than that of washcoated refractory metal oxidessuch as gamma aluminum oxide, it provides different distribution of thecatalyst 28. The catalyst 28 distributed over the anodized layer 24 arepartially shielded by the forthcoming washcoat layer 26 from the gasphase contaminants (e.g., catalyst poisons) which deactivate thecatalyst 28. This feature helps to increase the operating life of thecatalytic converter 18.

To create a rough texture on the surface of the anodized layer 24, theanodization is performed at a temperature above ambient. For example,the anodization can be performed at a temperature of about 40° C.

Next, the anodized core 22 is washcoated as indicated at steps 102 to108. A slurry is prepared, as indicated in step 102. The slurry is madepreferably from a jarmilled mixture of a refractory metal oxide and asynthetic silicone resin. A refractory metal oxide such as alumina,silica, aluminum silicate, magnesia, manganese oxide, titania, zirconiaor ceria may be used. A partially hydrated refractory metal oxide suchas boehmite may be used instead. The synthetic silicone resin may be amethylphenylsiloxane, a methylsiloxane or other organosiloxane resin inmonomeric or polymeric form. The resin may be in a concentrated form orit may already be diluted with an organic solvent. The slurry may bediluted (or further diluted) by adding an organic solvent such as analcohol (e.g., methanol, ethanol, isopropyl) or an aromaticsolvent(e.g., xylene, toluene). Slurry viscosity may be adjusted byadding additional solvent. The preferred slurry mixture containsboehmite, methylphenylpolysiloxane and toluene.

In the preferred slurry, the boehmite content in the slurry ranges from1 to 30 wt % and the methylphenylpolysiloxane content ranges from 1 to30 wt %. The remainder of the slurry is balanced by the solvent. Tofacilitate the cross-linking process, a small amount (e.g., <1 wt %) ofa zinc compound such as zinc octoate may be added to the slurry. Theviscosity of the slurry may range from 5 to 250 centipoise. Thepreferred viscosity of the slurry may range from 20 to 60 centipoise.

The slurry is then applied to the core 22, as indicated by step 104. Forexample, the core 22 may be dipped in the slurry. Following the dippingis an air knifing process to remove excess slurry. The highly adjustableviscosity of the washcoat slurry leads to a highly controllable coatingthickness from five to over sixty microns.

The core 22, coated with the slurry, is then dried and cured in a shortperiod of time from 2 to 10 minutes, as indicated at step 106. To drythe washcoat layer 26 in such a short period, the core 22 is rotatedwhile the washcoat layer 26 is being shaped, dried and cured by a highflow air knife. The air knife is aligned in a radial direction with theair flowing longitudinally through the core 22 (i.e., in the samedirection as that of the compressed air during operation of thecatalytic converter 18). The core 22 is rotated at a rate between 1 and5 rpm. During this step 106, the core 22 may be heated at about 80° C.to 250° C. for between ½ to 3 hours. This heat treatment both dries andcures the washcoat layer 26 and removes the organic solvent in thewashcoat layer 26. During this process, the organosiloxane resin reactsand cross-links the metal oxide particles among themselves as well aswith the oxide at the surface of the anodized layer 24. Resulting is astrongly bonded high surface area support that is formed via chemicalcross-linking between the metal oxide of the anodized surface layer 24and the organosiloxane resin and refractory metal oxide in the washcoatlayer 26.

For the monolith core 22 having fin elements 23, the washcoat layer 26generated from this process has very good thickness uniformity withineach fin element 23 and throughout the core 22. The slurry justdescribed dries faster than a slurry including a water-based binder. Thefaster drying allows the washcoat layer 26 to be applied more uniformlythan a washcoat layer formed from a slurry including a water-basedbinder.

Once the washcoat layer 26 has been dried and cured, the core 22undergoes calcination. Calcination, which is indicated at step 108, maybe performed between two to ten hours at above 300° C. The preferredcalcination temperature may range from 450° C. to 550° C. Duringcalcination, the organic material in the washcoat layer 26 is burnedoff. Also, the chemical bonds cross-linked during the curing stage aretransformed into a three-dimensional network of M1—[O—Si—O]_(n)—M2—Ochemical bonds, where n≧1 and M1 and M2 could be the same or differentmetals in the metal oxides in the anodized and washcoat layers 24 and26. The metal oxide in the anodized layer 24 is bridged with the metaloxide in the washcoat layer 26 through this network of bonds. Using thepreferred mixture of boehmite and organosiloxane and toluene on analuminum oxide anodized layer 24, the calcination also converts theboehmite to gamma phase alumina, and the organosiloxane in the washcoatlayer 26 is transformed into silica or reacts with alumina to formalumina silicate. Therefore, the resulting washcoat over the anodizedfin elements 23 consists mainly of gamma alumina, silica and aluminasilicate.

During calcination, the washcoat layer 26 retains its high surface area.Consequently, the high surface area provides high exposure andinteraction between the air and the catalyst.

After the washcoat calcination has been completed, the catalyst 28 isimpregnated in the anodized and washcoat layers 24 and 26, as indicatedat step 110. If the catalyst is bimetallic, the layers 24 and 26 may becatalyzed through a co-impregnation procedure, wherein the washcoatedcore 22 is dipped in a metal precursor solution. Preferably, the layers24 and 26 are co-impregnated with a metal precursor solution of nitratesalts of palladium and nickel. In place of palladium, a salt solution ofa metal from the precious metal group including platinum, rhodium, gold,iridium or silver may be chosen as the metallic precursor forimpregnation. Likewise, in place of Ni, a metal from the transitiongroup including manganese, cobalt, iron, or copper may be chosen. Theprecious and transition metals set forth above are typical of the metalsthat have been found to provide satisfactory results. However, othermetals may be used. The chosen metals are preferably combined in asolution containing an organic acid that maintains a suitable acidityand acts as a dispersion reagent. Such organic acid may include citricacid, acetic acid or tartaric acid.

After the catalyst has been impregnated, a catalyst calcination isperformed, as indicated at step 112, to convert the catalyst precursorsinto the active catalytic ingredients. The calcination temperature mayrange from 200° C. to 500° C. over 1 to 4 hours.

During catalyst impregnation, the dense portion of the anodized layer 24prevents the catalyzing reagent from attacking the metal of the core 22.This allows the catalyst 28 to be impregnated after the washcoat layer26 has been fully formed. Resulting is maximum catalyst utilizationwhich leads to longer lifetime.

This invention is further described, although not limited, by thefollowing examples.

EXAMPLE I

A set of aluminum fins with a volume of 0.385 ml and thickness of 7 milswas anodized in oxalic acid to form an aluminum oxide layerapproximately 10 microns thick. The set of fins was further washcoatedwith a jarmilled mixture of diluted synthetic silicone resin withboehmite. The fins were dried, cured and calcined. The final weight ofthe fins increased by 20% due to formation of a layer of gammaalumina/silica over the fin surface. The surface area of the washcoatedfins was about 40 to 70 m²/g including the weight of the fins. This finswere subsequently catalyzed in a Pd and Ni nitrate solution containingcitric acid through a co-impregnation process. This resulted in thefinal catalyst loading on the fins of 50 gPd/ft³ and 300 gNi/ft³. Thefins were tested in a reactor from 75° F. to 400° F. with a 1,000,000gas hourly space velocity (GHSV). Excellent ozone destruction activitywas observed.

EXAMPLE II

An aluminum alloy monolith having an offset fin construction wasanodized in oxalic acid to form an aluminum oxide layer approximately 10microns thick. The monolith was washcoated with a jarmilled mixture ofdiluted synthetic silicone resin (with methylphenylpolysiloxane as themain ingredient), and boehmite. After drying, curing and calcination atapproximately 538° C., the weight of the fins was increased by 20% dueto formation of a coating of gamma alumina/silica over the fins. Themonolith was subsequently catalyzed in a Pd and Ni nitrate solutioncontaining citric acid by a co-impregnation process. This resulted inthe final catalyst loading on the fin equal to 50 to 300 gPd/ft³ and 100to 500 gNi/ft³. The monolith was then tested in dry air containing 1.0ppm ozone at 2,000,000 GHSV. The pressure of test flow input was 25 psigand the temperature was 365° F. The ozone catalytic destructionefficiency was found to be 80% and above. The catalyzed core alsodemonstrated excellent resistance to known catalyst poisons, such assulfur compounds and aviation lubricant.

EXAMPLE III

An alumina alloy monolith having an offset fin construction was preparedper EXAMPLE II. The monolith was tested in a stream of 1 ppm ozone inair at 250,000 GHSV and 45 psig. The following excellent efficiency wasobserved:

T (° F.) Efficiency (% of ozone removed) 200 98.1 250 98.5 300 98.6 35098.7

EXAMPLE IV

This example demonstrates the excellent low-temperature efficiency ofozone conversion. A set of fins was removed from the monolith followingthe test in EXAMPLE III and tested in a stream of 2.5 ppm ozone in airat 1,000,000 GHSV and 3 psig. The following efficiency was observed

Normalized Efficiency (% of ozone removed/% of T (° F.) ozone removed at350° F.) 122 74 212 84 302 92 350 100

EXAMPLE V

Several analytic techniques were employed to characterize the propertiesof the washcoat layer and the catalyst prepared per EXAMPLE I. TheBrunauer-Emmett-Teller (BET) method was employed to determine thewashcoat surface area and pore size distribution. It was determined thatthe washcoat layer had the preferred surface area ranging from 200 to350 m²/gram (excluding the weight of the fins). The preferred porevolume of the washcoat ranged from 0.2 to 0.9 cm³/gram excluding themetal substrate. The average pore size ranged from 3 to 25 nm. Ascanning electron microscopic (SEM) imaging method was used to study themorphologies of the anodized layer and the washcoat layer. The SEMrevealed that the washcoat layer was highly porous while the anodizedlayer had a rough surface texture at the interface with the washcoatlayer and was dense at the interface with the aluminum core. These arepreferred morphologies for promoting catalytic decomposition of ozoneduring the operation and for providing strong bonding between thewashcoat and anodized layers with the metal substrate during thecatalytic operation as well as catalyst preparation. A high degree ofmetal and metal oxide dispersion over the washcoat and anodized layerswas verified by X-ray diffraction (XRD) and CO adsorption isothermmethods. For example, the CO adsorption isotherm measurement indicatedthat the Pd in the catalyst had the dispersion index greater than 50%.Not bound to any catalytic theory, it is commonly believed that theoxidative state of the catalyst metal is important to the ozonedestruction activity. To identify the oxidative state of the metal inthe catalyst, an X-ray absorption near edge structure (XANES) method andan X-ray photoelectron spectroscopic (XPS) method were employed. It wasdetermined that both metals had the preferred oxidation state for highcatalytic activity. For example, XANES and XPS revealed that both Pd andNi were in Pd⁺² and Ni⁺² oxidation states at the surface as well asthrough the depth of washcoat and anodized layers. Again not bound toany catalytic theory, it is commonly believed that the close interactionand the synergetic effect between the two catalyst metal ingredients areimportant to the ozone destruction activity and to extended lifetime.Extended X-ray absorption fine structure (EXAFS) spectroscopic methodand the scanning transmission electron microscopy (STEM) method wereused to investigate the short-range structure and the interactionbetween the metallic components. It was determined that the catalystmetals had a preferred short-range structure and close interaction. Forexample, EXAFS study indicated that Pd was coordinated withapproximately four oxygen atoms and STEM demonstrated that Pd and Niwere in close proximity, evenly distributed within the metal oxidemicro-crystallites throughout the catalyzed washcoat.

EXAMPLE VI

To demonstrate that the anodized aluminum layer provides improvedbinding strength for the washcoat layer and better corrosion resistanceduring the catalyzing, a pair of aluminum plates A and B havingdimensions of 1′×2′×{fraction (1/16)}′ was prepared. The first plate Awas cleaned with an industrial detergent followed by rinsing withdeionized water. The second plate B was subjected to the anodization inthe oxalic acid to form an anodized layer, as described in EXAMPLE I.Both plates were washcoated and subsequently catalyzed following theprocedure described in EXAMPLE I. An American Society for TestingMaterials (ASTM) test method for measuring adhesion (Designation D 3359)was used to evaluate the adhesion strength between the washcoat layerand the aluminum substrate for both plate A and plate B after thecatalyzing process. It was found that the washcoat layer on the secondplate B had much superior adhesion strength (classification was between3B to 4B) than the washcoat layer on the first plate A (classificationof less than 1B).

EXAMPLE VII

A thermal cycling test was conducted for an ozone destruction catalyticmonolith prepared according to the procedure in EXAMPLE II to evaluatethe mechanical durability of the washcoat layer and the catalyst underrepeated thermal stress condition. The monolith was subjected tocontinuous airflow with inlet air pressure of approximately 100 psig anda flow rate of 100 lb/minute. The temperature of the air was variedbetween 150° F. to 500° F. with a temperature ramp rate of 75° F./minute. A total of 2000 thermal cycles was conducted during the test.The washcoat layer and catalyst weight loss was measured at the end ofthe test cycle. It was found that the washcoat layer and catalyst hadexcellent mechanical resistance to thermal stress and that the totalweight loss was less than 2% after the test.

EXAMPLE VIII

A vibration test was conducted for an ozone destruction catalyticmonolith prepared per EXAMPLE II to evaluate the mechanical durabilityof the washcoat layer and the catalyst under severe vibrations. Themonolith was mounted on a vibration test stand and was subject to 15hours of random vibrations in both longitudinal and transversaldirections. Frequency was 10 Hz to 2000 Hz and 4.0 g rms in thelongitudinal direction and 10 Hz to 2000 Hz at 9.54 g rms in thetransveral direction. The mechanical integrity of the monolith wasinspected, and the washcoat layer and catalyst weight loss were measuredat the end of the vibration test. It was found that the monolith had nostructural damage or variation. Furthermore, it was found that thewashcoat layer and catalyst had excellent mechanical resistance tovibrational stress and that the total weight loss is less than 1 % afterthe test.

The invention is not limited to the specific embodiments above. Forexample, the washcoat layer may be applied directly to an unanodizedcore. Such a core may be made of stainless steel or titanium that hasbeen baked or otherwise oxidized at the surface. If the core is made ofa material that already has a porous or oxidized surface, the washcoatlayer may be applied directly to the surface of such a core.

The invention is not limited to the washcoat layer 26 described above. Awashcoat layer including a metal oxide mixed with a water-based binder(e.g., alumina sol, silica sol) may be applied to the anodized surfacelayer 24. However, such washcoat layers may not display the superiorcharacteristics of the washcoat layer 26 described above.

The catalyst 28 is not limited to any particular type and composition.For example, the catalyst 28 may be monometallic or bimetallic, or itmay include more than two metals. Therefore, the catalyst may include atleast two precious metals. It may also include at least one transitionmetal.

The core 22 is not limited to a single washcoating. Several washcoatsmay be performed to control uniformity of the washcoat layer 26.

Therefore, the invention is not limited to the specific embodimentsabove. Instead, the invention is construed according to the claims thatfollow.

What is claimed is:
 1. An ozone-destroying catalytic converter for anenvironmental control system of an aircraft, the converter comprising: acore; an anodized surface layer formed from a portion of the core; awashcoat layer on the anodized layer; and an ozone-destroying catalystimpregnated in both the anodized layer and the washcoat layer, loadingof the catalyst in the washcoat layer being higher than the loading ofthe catalyst in the anodized layer.
 2. The catalytic converter of claim1 wherein the anodized layer has a rough surface texture at an interfacewith the washcoat layer.
 3. The catalytic converter of claim 1, whereinthe catalyst includes at least one precious metal and at least onetransition metal.
 4. The catalytic converter of claim 1, wherein theanodized layer is an aluminum oxide, and wherein the washcoat layerincludes primarily gamma alumina, silica and alumina silicate.
 5. Thecatalytic converter of claim 1, wherein the anodized layer isapproximately 5 to 10 microns in thickness, and wherein the washcoatlayer is approximately 5 to 60 microns in thickness.
 6. The catalyticconverter of claim 1, wherein the washcoat layer alone has a surfacearea ranging from about 200 to 350 m²/gram.
 7. The catalytic converterof claim 1, wherein pore volume of the washcoat layer ranges from about0.2 to 0.9 cm³/gram, and average pore size ranges from about 3 to 25 nm.8. The catalytic converter of claim 1, wherein the anodized layer has adifferent morphology than the washcoat layer.
 9. The catalytic converterof claim 1, wherein the core is a monolithic core including a pluralityof offset fins providing a tortuous air flow path, the fins carrying theanodized and washcoat layers.